THE White House was put into lockdown after a car struck a security barrier.

The US Secret Service tweeted that the vehicle "did not breach the security barrier of the White House complex".

The agency added that the female driver of the vehicle was "immediately apprehended".

Witnesses saw a commotion on the White House grounds as security officers responded.

US President Donald Trump is hosting Prime Minister Malcolm Turnbull at the White House.

Mr Turnbull is remaining at the White House as the incident is being investigated.

TRUMP, TURNBULL DISCUSS NORTH KOREA, CHINA

Earlier Mr Trump, standing alongside Mr Turnbull at a White House press conference, issued North Korea with a military threat and encouraged the Australian Navy to join in freedom of navigation exercises in the South China Sea.

Mr Trump and Mr Turnbull put on a mostly united front at the White House on Friday with the leaders exchanging multiple compliments.

It was another major step forward since their contentious phone call last year that ended with Mr Trump abruptly cutting short the planned one hour chat after just 24 minutes and calling a refugee deal agreement "dumb".

They did, although politely, agree to disagree on gun control and the Trans-Pacific Partnership free trade agreement.

"Prime Minister Turnbull, it has been a pleasure hosting you today," Mr Trump said during the press conference, attended by Australian and US dignitaries, including former world number one golfer Greg Norman.

Hours before Mr Turnbull and wife Lucy pulled up to the South Lawn entrance of the White House and were greeted by Mr Trump and First Lady Melania, the president's administration announced "the largest set of sanctions ever imposed in connection with North Korea".

Mr Trump said if the sanctions did not work and North Korea continued to pursue nuclear weapons he might launch "phase two".

"Phase two may be a very rough thing," Mr Trump said.

"Maybe very, very unfortunate for the world."

Mr Trump acknowledged Australia's support in attempting to tame the North Korean regime.

"Australia is one of our closest partners in our campaign of maximum pressure to denuclearise the Korean Peninsula," he said.

Mr Turnbull talked up Mr Trump's relationship with Chinese leader Xi Jinping after observing them together at international conferences.

The prime minister also spoke positively about Australia's relationship with China.

"For our own part we see China's rise as being overwhelmingly a positive for the region and the world," Mr Turnbull said.

Mr Trump will likely anger China with his enthusiasm for the Australian Navy joining the US in conducting freedom of navigation exercises in the South China Sea.

Australia has so far declined to do that.

"We would love to have Australia involved and I think Australia wants us to stay involved," the president said.